This invention relates to the art of nuclear reactors and in particular to a bridge structure serving as a thermal and hydraulic barrier between heated outlet sodium and relatively cool sodium which has not flowed through the core. A nuclear reactor includes a pressure vessel into which a heat-transfer fluid, typically liquid sodium for fast-breeder reactors, is pumped under pressure. The sodium flows through the core and is heated; the hot sodium emerges from the vessel and flows to electrical power generating equipment. Typically sodium is introduced into the lower portion of the pressure vessel and flows upwardly through the core into the upper portion of the pressure vessel. Thus a thermal and hydraulic barrier is required in the pressure vessel between the core and the pressure vessel, separating the cool inlet sodium from the hot outlet sodium. Functionally, this barrier separates the annulus between the core and the pressure vessel -- containing low-pressure, low-temperature sodium -- from the upper sodium outlet plenum -- containing high-temperature, high-pressure sodium -- and provides elastic seals at inner and outer circumference. In addition, for a large fast-breeder reactor, it must provide guidance for the upper end of fuel and transfer storage chambers and provide the insulation required to minimize the heat flux from the hot outlet plenum sodium to the fuel storage annulus sodium.
In the Fast Flux Test Facility -- now under construction in the State of Washington -- Inconel is employed as the material of construction for this barrier and the barrier is not sealed to the pressure vessel at its outer edge. Flow experiments for the Fast Flux Facility indicate that flow instabilities and variable pressure distribution exist in the sodium pool immediately above the barrier. These conditions induce alternate coolant flow from the outlet plenum and from the cooler sodium below the barrier across the edge of the horizontal baffle forming this barrier. Inconel was selected as being the least expensive material which is capable of existing in this environment for the life of the reactor. In order to employ austenitic stainless steel, washing across the edges of the baffle by coolant of substantially different temperatures must be minimized. Due to the size of the baffle and the thermal stresses involved, this cannot be accomplished simply by adding a seal to the outer circumference of the barrier.